288 SURVEY OF THE COAST 



raise or lower the arm or radius of the instrument wliich it 

 directs ; and the inner steel screw having smaller threads 

 than the outer c', the effect of its motion is proportional 

 only to the difference between the distances of the thieads 

 of the two screws, by which means a very delicate motion 

 is obtained. 



By the three cones 6f, e, e, the instrument is fixed upon a 

 solid stand of pine wood, the circular top board of which is 

 two inches thick. A hexagonal frame underneath joins this 

 board to the three legs, and at the same time preserves the 

 top from warping. Pine wood was preferred, l)ecause it 

 warps the least ; and though, strictly speaking, a triangular 

 board would be sufficient, the circular one is much prefera- 

 ble, on account of the protection wliich it affords to the in- 

 strument against accidental touches in passing round it dur- 

 ing the observation. The board is, for the same reason, 

 three feet in diameter ; and I even took the precaution of 

 placing the cones e always above the legs of the stand, to 

 give the instrument more firmness. In the centre of the 

 board is a hole, so that the centre of the instrument may be 

 centered to the station by a plumbline hung from a loop in 

 the centre below the axis. 



The hollow hexagonal centre piece e above mentioned 

 receives through its middle the bell metal axis f, Plate V. 

 fig. 2 65 3, eleven inches in length, two inches in diameter 

 at the bottom of the inner hollow part of the piece c, and 

 an inch and a half at the top. which shows above the drum 

 in the perspective view of the instrument, Plate V. The 

 bottom part of this axis has a shoulder hh by which it is 

 fitted into the hexagonal centni piece, and below it the plate 

 gg projects to the outer circumference of the same hexagon, 

 which serves to fasten the axis to this piece, by means of 

 six strong screws, seen in fig. 8. The circular liole in the 

 bottom of the centre piece c is the centre upon which the 

 circle was divided. 



The brass socket of the axis, by which the whole upper 

 part of the instrument revolves upon the above axis, readies 



